Photophysical and Spectroscopic Investigations on (Oligo)Thiophene

Nov 17, 2009 - Department of Chemistry, University of Coimbra, Rua Larga, ... Present address: Cambridge Display Technology Ltd., Greenwich House, ...
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J. Phys. Chem. B 2009, 113, 15928–15936

Photophysical and Spectroscopic Investigations on (Oligo)Thiophene-Arylene Step-ladder Copolymers. The Interplay of Conformational Relaxation and On-Chain Energy Transfer J. Pina,*,† J. Seixas de Melo,*,† H. D. Burrows,† T. W. Bu¨nnagel,‡,§ D. Dolfen,‡ C. J. Kudla,‡ and U. Scherf‡ Department of Chemistry, UniVersity of Coimbra, Rua Larga, 3004-535 Coimbra, Portugal, and Makromolekulare Chemie, Bergische UniVersita¨t Wuppertal, Gaustrasse 20, D-42097 Wuppertal, Germany ReceiVed: June 9, 2009; ReVised Manuscript ReceiVed: October 13, 2009

An optical spectroscopy and photophysics study on four (oligo)thiophene-phenylene and (oligo)thiophenenaphthylene step-ladder type copolymers in solution (room and low temperature) and in the solid state (thin film) is presented. The study involves absorption, emission, and triplet-singlet difference spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation), excited-state lifetimes, and singlet and triplet energies. The overall data allow for a determination of the rate constants for all decay processes and from these several conclusions could be drawn: (1) in solution the main deactivation channels are radiationless processes (S1∼fS0 internal conversion and S1∼fT1 intersystem crossing); (2) from time-resolved fluorescence decays in the picosecond time domain three decay components are seen: a fast decay (10-20 ps) at short wavelengths, which becomes a rising component at longer wavelengths, an intermediate decay component (120-190 ps) most probably associated to isolated conjugated segments, and a third exponential related to the emission of the fully relaxed polymer. The assignment of the fast decay component to an on-chain energy transfer/migration is based of the dependence of the decay time on the solvent viscosity in combination with the investigation of an oligomeric model compound. Here, the absence of any significant changes of the decay parameters (decay times and preexponential factors) upon going from a less (toluene) to a more viscous (decalin) solvent together with the monoexponential fluorescence decay of the oligomeric model compound allow us to differentiate between deactivation of the singlet excited state by conformational relaxation and on-chain energy/transfer migration. Introduction The demand nowadays for efficient organic photovoltaic devices, displaying high energy harvesting efficiency and high charge carrier mobility has been the stimulus for a great deal of research, mainly driven by the tunable electronic properties of the organic semiconducting materials, the possibility of large area solution processing and the potentially low fabrication costs.1,2 Hereby, the conjugated donor and acceptor materials are key components toward high efficiency devices with high charge carrier mobility and a good coverage of the solar spectrum.3-5 The optical and electronic properties of organic π-conjugated polymers vary significantly with their main chain conformation. In single-stranded polymers such as polythiophenes and polyphenylenes twisting around the inter-ring single bonds of the conjugated main chain decreases the degree of conjugation (and the so-called effective conjugation length), thus leading to drastic changes in the optical and electronic properties.6-8 One approach to circumvent this problem is the incorporation of the π-conjugated system into a rigid and planar ladder-type framework (with its negligible conformational flexibility) leading to a maximum degree of conjugation. The intermediate term “stepladder polymer” stands for a partial ladderized structure and is * To whom correspondence should be addressed. E-mail: (J.P.) [email protected]; (J.S.d.M.) [email protected]. Fax: 00351 239 827703. † University of Coimbra. ‡ Bergische Universita¨t Wuppertal. § Present address: Cambridge Display Technology Ltd., Greenwich House, Madingley, Road, Madingley Rise, Cambridge, CB4 0TX, UK.

for poly(para-phenylene)s associated with the presence of ladder segments of a given number of bridged phenylene rings per repeat unit. The partial planarization of adjacent phenylene rings results in an enhanced conjugative interaction along the backbone if compared to their single-stranded counterparts, while the bridging carbon atoms offer the possibility to attach solubilizing alkyl chains without hampering the π-conjugation by ortho-substitution effects. Well-described step-ladder poly(para-phenylene)s include poly(tetrahydropyrene)s and poly(9,9dialkyl)fluorenes that have already been demonstrated to have a huge application potential.9 The limited conformational freedom of ladder and stepladder polymers decreases the inherent conformational disorder which often reduces the electron delocalization within these π-conjugated systems. For ladder poly(paraphenylene) (LPPP) it was found that only a very low concentration of active exciton traps (e.g., defects or impurities) is present, and as a consequence they display a high charge carrier mobility and intense solid state photo- and electroluminescence.6,10,11 In addition, the rigidity of the structures leads to geometrically very similar ground and excited states, and in contrast to other poly(para-phenylene)s6,12 there is very little Stokes-shift between the absorption and emission. The ladder approach is very useful in maximizing the extent of on-chain π-conjugation. This is also valid for conjugated polymers with lower band gap energy that could be applied as donor component in organic solar cells. For phenylene-type ladder polymers (LPPPs), a well-established way of shifting the absorption band to lower excitation

10.1021/jp9054022 CCC: $40.75  2009 American Chemical Society Published on Web 11/17/2009

(Oligo)thiophene-arylene Step-ladder Copolymers SCHEME 1

J. Phys. Chem. B, Vol. 113, No. 49, 2009 15929 j n), TABLE 1: Mean Number Average Molecular Weight (M j W), Polydispersity (D), Weight Average Molecular Weight (M and Degree of Polymerization (DP) for the Copolymers polymer

jn M

jW M

D

DPa

BTB BTBR1 BTBR2 BTNp

7600 15000 14000 14000

15700 46000 27000 27500

2.07 3.07 1.93 1.94

22 57 30 44

a

energies is the replacement of para-phenylene by electronrich heteroarylene moieties, such as 2,5-thienylene.6,12 This results from the lower aromatic resonance energy of the heteroaromatic thiophene building block when compared to that of benzene.13 In this study, the electronic spectral and photophysical properties of partially rigidified (oligo)thiophene-phenylene and oligo(thiophene)-naphthylene step-ladder copolymers were investigated in solution at room (293 K) and low temperature (77 K) as well as in the solid state. The structures and acronyms of the samples studied are depicted in Scheme 1. These alternating copolymers consist of a symmetrically functionalized bridged dithienylphenylene unit combined with short (oligo)thiophene segments of different length (0-2 thiophenes), see Scheme 1. The replacement of the central 1,2,4,5-tetrasubstituted phenylene core of BTB by a 1,2,5,6-tetrasubstituted naphthylene moiety in BTNp was also investigated (Scheme 1). The introduction of decylphenyl or tert-butylphenyl side branches was necessary to ensure sufficient solubility and processability of the copolymers from solution. Experimental Section Synthesis, purification, and basic characterization of the investigated copolymers have been described elsewhere.14 Absorption and fluorescence spectra were recorded on Shimadzu UV-2100 and Horiba-Jobin-Ivon SPEX Fluorog 3-22 spectrometers, respectively. The fluorescence spectra were corrected for the wavelength response of the system. The

j W. Based on M

fluorescence quantum yields were measured using pentathiophene (φF) 0.33 in methylcyclohexane)15 as standard. Fluorescence decays were measured using a home-built TCSPC apparatus described elsewhere16,17 and were analyzed using the modulating functions method of Striker.18 The experimental setup used to obtain triplet absorption spectra and triplet yields has been described elsewhere.16,19 Good first-order kinetics were observed in all cases for the decay of the lowest triplet state. The triplet molar absorption coefficients were obtained by the energy transfer method20 and were determined using pyrene, εT ) 20 900 M-1 cm-1 (420 nm),21 as triplet energy donor. The concentrations for the compounds studied were 10-5 M (in terms of repeat units) and they were dissolved in a solution of pyrene (10 mM) in toluene. Details on the experimental procedures and equations used can be found in ref 19. The intersystem crossing yields for the compounds (φT) were obtained by comparing the ∆OD at 525 nm of benzene solutions optically matched (at the laser excitation wavelength) of benzophenone (standard, φT ) 1 in benzene21) and of the compound, as described elsewhere.19 Triplet states were also characterized by pulse radiolysis using the Free Radical Research Facility, Daresbury, UK. The experimental setup used in these experiments has already been described in detail.22 All solutions were bubbled with argon for about 30 min before experiments. Room-temperature singlet oxygen phosphorescence was detected at 1270 nm with equipment and procedures elsewhere reported.16,19 From these signals, the singlet oxygen quantum yields were obtained22 by comparison with the reference compounds 1H-Phenalen-1-one (φ∆ ) 0.93 in toluene23) and Rose Bengal (φ∆ ) 0.76 in methanol24). Thin films from the compounds were obtained with a Desktop Precision Spin Coating System, Model P6700 Series from Speedline Technologies with procedures reported elsewhere.19 The fluorescence emission spectra of the thin films were obtained with a Horiba-Jobin-Yvon integrating sphere. The solid-state photoluminescence quantum yields in thin films were obtained as described previously.19,25,26 Results The molecular weight data of the copolymers, including the j n), the weight average mean average molecular weights (M j n) and j W/M j molecular weights (MW), the polydispersity (D ) M the degree of polymerization (DP) are presented in Table 1. j n were found to be in The mean average molecular weights M the range 7600-15 000 g mol-1 with polydispersities between j W data, we obtained degrees 1.93 and 3.07. On the basis of the M of polymerization ranging between 22 (BTB) and 57 (BTBR1); see Table 1. Singlet State. Figure 1 presents the absorption and fluorescence spectra of the investigated step-ladder copolymers in toluene at room (293 K), low temperature (77 K), and as thin

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Pina et al. temperature absorption spectra. Upon going to 77 K, a further narrowing of the spectra and an increase in the spectral resolution was observed. In thin films, a broadening of the emission spectra is observed thus leading to a reduced vibrational structure probably due to the occurrence of intermolecular interactions in the solid state. The room-temperature absorption and emission spectra of the dithienyl-substituted dithienylphenylene (DTB) step-ladder model compound (see Scheme 1 and Figure 1) in toluene displays absorption (λmax) 425 and 442 nm) and emission (λmax) 467 and 498 nm) maxima (see Table 2) at shorter wavelengths if compared to the corresponding copolymers. The absorption spectra of the oligomer (Figure 1) displays a well-resolved vibrational structure that is indicative of the rigid structure for this compound.14 The observed red shift in the absorption and emission bands, upon going from the oligomer to the copolymers (BTB, BTR1, and BTR2) illustrates that the conjugation segment in the copolymers involves more than one repeat unit. Time-Resolved Fluorescence. Fluorescence lifetimes of the copolymers in solution were initially obtained with time resolution of ∼150 ps and the results suggested a single exponential decay. However, when the decays are collected with better time resolution (∼3 ps)17 these are now best fitted with triexponential decay laws according to eq 1:

Iλ(t) )

∑ aije-t/τ

j

(1)

ij

Figure 1. Absorption and fluorescence emission spectra for the copolymers in toluene at 293 K, 77 K (from excitation spectra in the case of absorption), and in thin films. Also present at 293 K are the normalized absorption and fluorescence emission spectra for the dihexylthienyl-substituted bis(2-thienyl)phenyl oligomer in toluene.

films. At 293 K, the absorption spectra are relatively broad with a main absorption maximum accompanied by a low energy absorption shoulder (Figure 1). This behavior is somewhat in contrast to fully planarized para-phenylene ladder polymers, where highly structured absorption bands could be observed.27,28 In the room temperature absorption solution spectra a red shift of ca. 25 nm is seen when going from BTB to BTBR1; see Figure 1 and Table 2. In contrast to the often observed behavior (gradual red shift of the spectra29), the introduction of an additional thiophene in BTBR2 leads to a negligible, further red-shift. Upon cooling from room temperature (293 K) to low temperature (77 K), a slight red shift of the long wavelength absorption band and a much more pronounced vibrational structure was detected (see Table 2). For the solid state (thin films), a broadening of the absorption bands (compared with solution at 293 K) is observed (Figure 1). No significant changes were seen in the absorption maxima after replacement of the central phenylene in BTB by a naphthylene (BTNp) moiety, although a narrowing of the band and a slight increase in the vibrational structure was perceptible (Table 2 and Figure 1). This was previously attributed to ongoing aggregation of the (less soluble) copolymer BTB in toluene mirrored by the broadening of the longest wavelength absorption band.14 For BTNp, the characteristic naphthalene absorption/emission feature could not be detected thus indicating that the naphthalene is in full conjugation with the main-chain chromophoric system. The fluorescence emission spectra of the copolymers at 293 K (Figure 1) are more structured when compared to the room

where aij (i ) 1, 2, 3; j ) 1, 2, 3) are the pre-exponential factors and τj are the decay times; see Figure 2. The decay time values obtained with nanosecond time resolution (150 ps) were in good agreement with the longest decay time values obtained with picosecond time resolution. The fluorescence decay times were also collected as a function of the emission wavelength (Tables 3 and 4) and in solvents of different viscosity (Figures 2B and 3). Triplet State. The triplet-triplet transient absorption spectra (as triplet-singlet difference spectra) were obtained in solution at 293 K using the laser flash photolysis technique. In addition to ground state depletion, the transient spectra show an intense absorption with bands between 500 and 800 nm, Figure 4A. Figure 4A shows a slight, but significant red shift of the transient T1fTn absorption spectra with increasing thiophene content from BTB to BTBR2. This observation agrees with findings on oligothiophenes15,30 but contrasts to the behavior of fluorenethiophene alternating copolymers (with 1-3 thiophene units).29 The similar triplet-triplet absorption features for the fluorenethiophene alternating copolymers29 have been attributed to strongly localized triplet states. This, together with the relatively broad triplet-triplet absorption spectra of the copolymers (Figure 4A), suggests a somewhat increased delocalization of the triplet states for the investigated step-ladder copolymers due to an increased effective conjugation along the copolymer backbone. For BTB and BTBR1, the spectra and energies of the triplet states were also investigated by the pulse radiolysis energy transfer method in argon saturated benzene solution in the presence of biphenyl as sensitizer, Figure 4B. Similar transient absorption bands were observed as those obtained by the laser flash photolysis method, except at longer wavelengths (>700 nm), where the flash photolysis spectra appear to be attenuated by the diminished response of the detector. Although recent papers28 have reported weak phosphorescence for ladder polymers, such as methyl-substituted ladder-

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TABLE 2: Spectroscopic Data for the Ladder-Type Copolymers in Toluene at Room Temperature (293 K), Low Temperature (77 K), and in the Solid State (Thin Films) (the Underlined Wavelengths Are the Band Maxima) compound

Abs λmax (nm) 293 K

Abs λmax (nm) 77 K

Abs λmax (nm) film

Fluo λmax (nm) 293 K

Fluo λmax (nm) 77 K

Fluo λmax (nm) film

T1fTn λmax (nm) 293 K

εTTa (M-1 cm-1)

∆SSb (cm-1) 293 K

BTB BTBR1 BTBR2 BTNp

480 505 508 482

470, 505, 540 520 525, 558 490, 525

495 510 515 485

562, 600 569, 615 572, 620 533, 570

569, 612 578, 627 580, 630 536, 577

563, 590 610 615 535, 572

660 690 750 660

11280 13050 15180 7580

3040 2227 2203 1985

a

Triplet molar extinction coefficient. b Stokes-shift.

Figure 2. Room temperature fluorescence decays for BTB in toluene and decalin obtained with (A) λexc ) 455 nm and (B) with λexc ) 396 nm and collected at λem ) 700 nm. For a better judgment of the quality of the fits, autocorrelation functions (A.C.), weighted residuals (W.R.), and chi-square values (χ2) are also presented as insets. The dashed line in each decay is the instrumental response function.

TABLE 3: Photophysical Properties Including Quantum Yields (Fluorescence, OF, Internal Conversion, OIC, Triplet Formation, OT, and Sensitized Singlet Oxygen Formation, O∆), Lifetimes (τF, τ0F, τT), Rate Constants (kF, kNR, kIC, kISC), Singlet (ES) and Triplet (ET) Energies, Together with the Singlet-Triplet Energy Splitting (∆ES1-T1) for the Ladder-Type Copolymers in Toluene and in the Solid State τFa 293 K

φFb 77 K

φF film

τFc (ns) film

kFd (ns-1) 293 K

kNRd (ns-1) 293 K

kISCd (ns-1) 293 K

φICd 293 K

kICd (ns-1) 293 K

τF0 d (ns)

φT

φ∆

τT (µs)

ESe (eV)

ETf (eV)

∆ES1-T1 (eV)

compound

φF 293 K

BTB BTBR1 BTBR2 BTNp

0.15 0.17 0.22 0.10

0.720 0.560 0.580 0.720

0.062 0.067 0.13 0.091

0.030 0.073 0.030 0.075

1.60 1.27 1.01 1.06

0.208 0.304 0.379 0.138

1.18 1.48 1.34 1.25

0.431 0.607 0.328 0.236

0.54 0.49 0.59 0.73

0.750 0.875 1.02 1.01

4.8 3.3 2.6 7.2

0.31 0.34 0.19 0.17

0.32 0.30 0.15 0.12

37 16 11 25

2.29 2.24 2.23 2.37

1.55 1.55 ND ND

0.74 0.69 ND ND

a The fluorescence decay times correspond to the longest decay time obtained with a time resolution of 3 ps. b In methylcyclohexane. c Major decay component obtained in thin films. d

kF )

φF 1 - φF 1 - φF - φT φT τF ; kNR ) ; kIC ) ; kISC ) ; φIC ) 1 - φF - φT ; τF0 ) τF τF τF τF φF

e Values taken from the intersection between the room temperature absorption and fluorescence emission spectra. f Values obtained by the pulse radiolysis energy transfer method; see Figure 4B.

type poly(para-phenylene), MeLPPP, and related step-ladder analogues with varying length of the ladder oligophenyl segments in frozen 2-methyltetrahydrofuran glasses, we were unable to detect any phosphorescence with our current experimental system. In the absence of phosphorescence, the pulse radiolysis-energy transfer technique31 was used to obtain the triplet energy values of BTB and BTBR1 (see Figure 4B and Table 3). In this case, anthracene (1.84 eV) and perylene (1.54 eV) were tested as

sensitizers (Figure 4B).21 Triplet state formation of BTB and BTBR1 was observed when anthracene was used as sensitizer showing that the triplet energy of the copolymers should be located below the T1 value of anthracene. With perylene (as triplet donor), we were unable to obtain the transient triplet-triplet spectra of the polymers showing that the triplet state energy value of the copolymers are located between those of anthracene and perylene (the kinetic traces suggest this is very close to perylene or probably just above). The triplet energies for BTB

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TABLE 4: Fluorescence Decay Times (τj) and Pre-Exponential Factors (aij) for the Ladder-Type Copolymers in Toluene at 293 Ka

plotting the characteristic initial singlet oxygen phosphorescence intensity as a function of the laser dose and comparing the slope of the curve with that obtained for 1H-Phenalen-1-one or Rose Bengal as standards (see Table 3 and Figure 5).24

compound λem τ1 (ps) τ2 (ps) τ3 (ps) BTB BTBR1 BTBR2 BTNp

530 560 670 540 565 670 540 565 670 530 620

20

190

720

20

160

560

20

130

580

10

120

720

ai1

ai2

ai3

χ2

0.519 0.310 0.050 0.601 0.387 0.263 0.603 0.210 0.032 0.477 0.232

0.121 0.198 0.217 0.126 0.215 0.211 0.124 0.210 0.173 0.140 0.198

0.360 0.492 0.733 0.273 0.398 0.526 0.274 0.580 0.795 0.382 0.571

1.09 1.08 1.04 1.11 1.13 1.14 1.15 1.18 1.03 1.06 1.08

a The data presented were obtained by global (simultaneous at the three emission wavelengths) analysis of the decays. Also presented are the χ-squared values for a better judgment of the quality of the fits.

Figure 3. Room temperature fluorescence decays for the dihexylthienyl substituted bis(2-thienyl)phenyl oligomer (DTB) in toluene and decalin obtained with λexc) 425 nm. For a better judgment of the quality of the fits, weighted residuals (W.R.), autocorrelation functions (A.C.) and χ2 values are also presented. The dashed line in the decays is the instrumental response function.

and BTBR1 in solution at room temperature are therefore estimated to be 1.55 ( 0.05 eV. Singlet oxygen formation quantum yields (φ∆) from aerated solutions of the step-ladder copolymers were determined by

Discussion We start this discussion with the most important target of this work: the analysis of the time-resolved fluorescence decays and its implications on the relaxation processes of the first singlet excited state in conjugated organic polymers. In general, the fluorescence decay times of the systems studied do not show any significant change when collected across the fluorescence spectra and can be considered to be independent of the emission wavelength (λem) over the whole range studied. However, the same is not true for the pre-exponential factors (aij), in agreement with a progressive spectral red shift. This allows the global analysis of the fluorescence decays. The time-resolved fluorescence behavior found for the copolymers under study contrasts with what was found for the fully planarized ladder polymer MeLPPP and related step-ladder poly(para-phenylene)s (with 3-5 phenyls in the ladder segments) in which single exponential fits were found in all cases.28,32 Analysis of the decay profiles for the copolymers investigated reveals the presence of a fast component (10-20 ps), an intermediate decay time of about 120-190 ps, and a slower decay time of around 560-720 ps (Table 4). As listed in Table 4, the pre-exponential factors are all positive, although, when the emission decays are collected at λem> 670 nm, a negative amplitude arises which is associated with the shortest decay time (see Figure 2B). This was investigated for BTB in toluene solution and the analysis of the decays collected in the tail of the emission band (700 nm, see Figure 2B) revealed the presence of a rise-time associated with the shortest decay time (ca. 10 ps). This fast component, which appears as a decay time in the high energy part of the (emission) spectra (Table 4 and Figure 2A) and as a rise time for low energies (Figure 2B), is usually associated with fast relaxation processes in the excited state manifold. In previous reports, these relaxation processes have been attributed either to excitation energy migration along the (co)polymer to lower energy conjugated segments17,33 or to conformational relaxation processes in the first singlet excited state.17,32,34-37 With the copolymers investigated here a distinction between the two mechanisms (energy migration and conformational relaxation) is required. For fully planar ladder polymers (such as MeLPPP), where conformational changes are not expected, only energy transfer between emissive segments of slightly different electronic nature should occur. Additional information is collected from the fluorescence emission decay of BTB in the more viscous solvent decalin (Figure 2B). Going to decalin, the fastest decay time (and its associated amplitude) remains effectively unchanged, within experimental error (see Figure 2B). Because conformational relaxation strongly depends, among other properties, on solvent viscosity34,35,38 we can conclude that this process is unlikely to be responsible for the shortest decay component. The single exponential decay times obtained for the model compound DTB (where energy migration is not possible) in toluene (τF ) 1.06 ns) and decalin (τF ) 1.12 ns) solution are in agreement with this finding (see Figure 3). The study of oligomeric model compounds, and in particular of their time-resolved fluorescence behavior in solvents with different viscosities and as a function of temperature, is often used to distinguish between the fast relaxation processes

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Figure 4. (A) Transient singlet-triplet difference absorption spectra for the polymers in toluene at room temperature. (B) Schematic picture of the pulse radiolysis energy transfer method to obtain the triplet energy of the copolymers.

Figure 5. Plots of the phosphorescence intensity of singlet oxygen at 1270 nm as a function of laser intensity for BTBR1 and Rose Bengal (reference) at 293 K

occurring in the excited state of π-conjugated polymers.17,32,34,38 Using this methodology, it has been seen that, for example, in the case of polyfluorene derivatives32 and p-phenylenevinylene oligomers,34,38 the marked viscosity dependence and thermal activation strongly supports the idea that conformational relaxation is responsible for the fast component in the emission decay while, in contrast, the data for alternating binaphthylthiophene copolymers17 indicate that an energy migration mechanism is operative. In the present case, the assignment of the fast decay to energy transfer can therefore be explained in terms of an energy migration from higher to lower excitation energy segments within the copolymer chain. The observation of a relatively broad, room-temperature absorption spectrum, a more structured and more red shifted fluorescence spectrum (see Figure 1), and in comparison to MeLPPP with its very small Stokes loss, corroborates this model of intrachain energy transfer to the energetically favored conjugated segments.32 Interchain processes can be ignored due to the low concentration of the copolymers used; that is, these can be considered as isolated copolymer chains. Further analysis of the fluorescence decays shows that the longest decay time can be attributed to the emission from the energetically relaxed species of the copolymers and its contribution (from the pre-exponential factors) increases with the

emission wavelength, in agreement with the dominance of emission from the lower energy relaxed species. The additional intermediate decay time may be associated with the emission of conformers with a shorter conjugation length. Indeed, the occurrence of conjugation barriers leading to an interruption of the conjugative interaction has already been reported in the literature for a nonplanar, ethylene-bridged poly(para-phenylene) ladder polymers (LPDP).39 The nonplanar geometry of the repeat units was also documented in the X-ray structure of the dithienyl-substituted dithienylbenzene model compound. Here, the central dithienylbenzene core adopts a planar geometry, the attached thienyl substituents show a slight out-of-plane twist of 20.05° relative to the planar core segment thus introducing an increased inhomogeneity of the electronic ground and excited states.14 A similar nonplanar structure is expected for BTB, BTBR1, BTBR2, and BTNp resulting in a weakly distorted backbone structure. Additional support for the occurrence of conformational disorder leading to geometrical differences between ground and excited states comes from the relatively large Stokes-losses (∆SS) from 1985 cm-1 for BTNp to 3040 cm-1 to BTB if compared to the very low ∆SS values (e150 cm-1) found for the fully planar ladder poly(para-phenylene) (LPPP) and ladder poly(1,4phenylene-2,5-thienylene) (LPPPT) polymers.6,12 For LPPP (where no rise time was found in the emission decay)28,32,40 and LPPPT, the small ∆SS value was attributed to the geometrically fixed, planar structure that stabilizes the backbone against torsional displacements during the S0fS1 electronic transition.12,27 This will be further considered in the analysis of the absorption and fluorescence spectra. Absorption and Fluorescence Spectra. We can make some important conclusions from the data of Table 2 and Figure 1. First, the less structured absorption bands at room temperature and in thin films can be interpreted, as for polyfluorenes, as resulting from contributions of segments with different conjugation lengths as a consequence of a relatively low energy barrier for an interconversion between different segments.32,37,41,42 In contrast, the better structured fluorescence spectra suggests that the emission is a consequence of the following. Well-structured emission spectrum is characteristic of molecules with rigid and well-defined excited state geometries. This is what happens with thiophene-like structures in the S1 state where the quinoidal-

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SCHEME 2

like structure confers to the excited state of the oligomer/polymer a rigid and delocalized structure (vibronic) similar to that found for polyaromatic hydrocarbons. The other factor, which is partially a consequence of the former, also contributing to a more defined conjugation length (involving the naphthalene and thiophene units) results from a less broad distribution of effective conjugation lengths present in the singlet excited state of these copolymers. Moreover, it is worth stressing that due to the polydispersity of these copolymers and different distributions of conformers in the ground and excited states, an ensemble/ distribution of polymers with different conjugation lengths exists and strongly contributes to the inhomogeneous broadening observed in the absorption spectra of these copolymers. The marked Stokes-shift values (Table 2) also indicate that either there is a greater effective conjugation in the excited state (driven by an activated relaxation process) or that an energy migration to more relaxed states occurs.32 The negligible red shift in the emission spectra on going from 293 to 77 K is in agreement with a relatively rigid structure in the excited state (Figure 1 and Table 2) thus indicating that there is no change in the structure of the emitting moiety upon cooling. A further relevant feature was observed with both the absorption and fluorescence excitation spectra in which a slight red shift and a more pronounced vibrational structure was observed upon cooling (Figure 1 and Table 2). This supports an increase in the planarity of the polymer chain at 77 K compared to 293 K. We believe that this observed red shift on cooling can be attributed to a greater population in the ground state of more planar conformers at 77 K enhancing the conjugation within the polymer backbone.15,43 In these copolymers, this may be associated both with a decrease in the twist angle between the building blocks and a reduction at low temperature of the rotational degree of freedom of the single bonds connecting the thiophene units.28 These will have a higher energy, and the absorption at 77 K will then be from a higher energy S0 state compared to 293 K but to the same S1 state (identical energy at 293 and 77 K) as illustrated in Scheme 2. We also note the insignificant red shift in the absorption and emission maxima when going from BTBR1 to BTBR2 (see Figure 1 and Table 2) confirming the dominating influence of the ladder dithienylbenzene moieties. Photophysical Behavior in Solution and in the Solid State. The photophysical parameters obtained are presented in Table 3. For the copolymers in solution, the radiationless processes (φIC + φT) constitute the main deactivation process for S1. The data in Table 3 allows the determination of the radiative (kF) and radiationless rate constants (kNR ) kIC + kISC), supporting this view. A significant decrease in the fluorescence quantum yield is observed upon going from the dithienyl substituted dithienylbenzene oligomer (φF ) 0.52) to the copolymers (0.10-0.22), see Table 3. This can be attributed to the increase in the degree of vibrational freedom in the excited state and a concomitant increase in the internal conversion decay channel upon going from the oligomer to the copolymers.

Pina et al. Analysis of Table 3 reveals a slight increase in the fluorescence quantum yield (φF) upon going from BTB (without thienyl units between the planar dithienylbenzene segments) to the copolymer having one and two additional thienyl moieties within the repeat unit, BTBR1 and BTBR2, respectively. This is in agreement with the behavior found for linear unsubstituted and oligothiophenes (Rn’s with n ) 1-6)15 naphthalene-oligothiophene44 copolymers in which an increase in φF was observed with the increase on the number of thiophene units. The opposite behavior was found for polyfluorene (PFO, φF ) 0.79)45 if compared with fluorene-(oligo)thiophene copolymers where a decrease in the fluorescence quantum yield was observed (φF ) 0.57 for PFaT and 0.38 for PFaTT)29 when increasing the number of thienyl units (from 1 to 2) in the copolymer. This was explained by an increase of the intersystemcrossing quantum yields (φT) induced by the heavy atom effect of the sulfur atom (spin-orbit coupling). In the present copolymers, a decrease in φT from 0.31 (BTB) to 0.19 (BTBR2) was observed, see Table 3. This decrease of the intersystem crossing yield shows that the spin-orbit coupling is not enhanced by the classical heavy atom effect since in that case we would expect a higher value for BTBR2 (in comparison to BTB). As has been discussed for various types of oligothiophenes,15,16,30,44 and probably also true in our case, the spin-orbit coupling is also mediated by a charge transfer (CT) singlet state mixing matrix elements involving the CT singlet donor and the triplet acceptor. Moreover, in the present case the decrease in φT (and apparent concomitant increase in φF) is not due to a significant increase in kF, but should result from a gradual reduction of the charge transfer character between the singlet and triplet states involved as the number of thienyl rings (n) increases. This may mirror a gradually smaller overlap of the electron donor-electron acceptor molecular orbitals with increasing n.15 The singlet energies (ES) for the copolymers are in the range 2.23-2.37 eV (Table 3). These values are similar to those of related fluorene-(oligo)thiophene copolymers PFaT (2.69 eV), PFaTT (2.55 eV), and PFaTTT (2.40 eV).29 The triplet energies (ET) for the copolymers BTB and BTBR1 have, within experimental error, the same value, 1.55 eV; Table 3. The resulting singlet-triplet energy splitting (∆ES1-T1) values (Table 3), which are 0.74 eV for BTB and 0.69 eV for BTBR1, are in good agreement with those typically found for conjugated polymers.31,46,47 It is worth noting that the singlet and triplet energy values for the copolymers are lower than those observed for oligothiophenes15,30 indicating a lower HOMO-LUMO energy difference for the copolymers due to an extended π-delocalization. Substitution of the phenylene group by a naphthylene in the planar core moiety (BTNp) decreases φF and φT with a concomitant increase in φIC (Table 3). This observation, together with the observed increase in the radiative lifetime (τF0 ) τF/φF) shows that the balance between the radiative and nonradiative decay processes is shifted toward the nonradiative processes when going from BTB to BTNp (Table 3). The decrease in φF contrasts to what was previously seen for naphthalene-(oligo)thiophenes, where an increase in this parameter was observed in comparison to oligothiophenes.15,30 No significant changes were seen in the singlet energy values when going from BTB (ES ) 2.29 eV) to BTNp (ES ) 2.37 eV) thus showing that a similar HOMO-LUMO energy splitting is observed. The decrease in φF going from BTB to BTNp is expected to be related to the increase in the density of vibronic states and

(Oligo)thiophene-arylene Step-ladder Copolymers vibrational overlap, with the introduction of naphthalene, thus increasing the internal conversion deactivation channel (as shown by the increase in φIC and kIC, see Table 3) between the first singlet excited state and the ground state. The decrease in the Stokes-shift (∆SS) values from 3040 cm-1 (BTB) to 1985 cm-1 (BTNp) and the spectroscopic behavior observed in the absorption and emission bands (see Table 2 and Figure 1) show that although there are significant differences in geometry when going from the ground state to the excited state of the copolymers this effect is less prominent for BTNp. The less pronounced geometrical changes, during the transition from the ground to the excited state of BTNp compared to BTB, seems to increase the radiationless coupling modes of the S1 and S0 states due to a better vibrational overlap. In general, for all the compounds investigated, the φF values are lower at 77 K than those obtained at 293 K (Table 3). This is in contrast to the general behavior found for oligothiophene derivatives, where an independence of φF upon temperature has been found.15,30 However, the observed decrease in φF with temperature is in agreement to what was previously seen for naphthalene-thiophene copolymers.44 Again, the small red shift in emission upon going from 293 to 77 K shows that a similar conformation is adopted at both temperatures, thus suggesting the occurrence of some aggregation at 77 K as a possible explanation for this decrease in φF. In general, aggregate formation increases the excited-state nonradiative decay pathway (by increasing the interchain and intrachain interactions between the polymer chains or within the polymer backbone) with the concomitant decrease of the radiative decay channel. In thin films, the φF values are lower than those observed at room temperature (Table 3). This is again the general trend found for π-conjugated systems when going from solution to the solid state and has been attributed to the increase in the contribution of radiationless processes.37 The decrease in φF and the (small) red shift in the absorption maxima, upon going from solution (293 K) to the solid state, agrees with what was found in solution at 77 K. Again, this can be attributed to the presence of intermolecular interactions (aggregation) between copolymer chains.41 Table 3 shows that the singlet oxygen formation yields are very close to the quantum yields for triplet formation, providing some support for the reliability of the latter values and indicating that the efficiency of triplet energy transfer to produce singlet oxygen (S∆ ) φ∆/φT) is very close to unity. These results suggest that the reaction with molecular oxygen may be an important pathway for the deactivation of the triplet state of these copolymers and may be relevant to their degradation in nonencapsulated devices. Comparison with All-Phenylene and Phenylene-Thienylene Ladder-Type Polymers. A marked decrease in φF is observed between MeLPPP (φF ) 0.79 in benzene solution)45 and the copolymers under study (Table 3). Although the chemical structure (considering fluorene, φF ) 0.68,21 as the building block of MeLPPP) and photophysical properties of MeLPPP differs from the copolymers investigated, nevertheless, and in agreement with the observed decrease in φF going from the model compound DTB (φF ) 0.52) to the copolymers, one of the reasons that could explain the greater importance of the fluorescence pathway for MeLPPP, compared with these copolymers, is the rigid planar MeLPPP structure, which reduces the vibrational degrees of freedom of the singlet excited-state thus favoring the radiative deactivation channel (as seen by the negligible value for internal conversion, φIC ≈ 0, found for MeLPPP45).

J. Phys. Chem. B, Vol. 113, No. 49, 2009 15935 Comparison of the absorption properties of the investigated step-ladder copolymers with those of the all-phenylene ladder polymer LPPP and the heteroarylene ladder polymer containing 2,5-thienylene units (LPPPT) leads us to the following conclusions: (i) the less structured absorption band of the copolymers contrasts with the very defined and structurally resolved π-π* transition bands observed for LPPP and LPPPT, thus indicating the nonplanar π-system of the copolymers with some conforabs of the compounds (Table 2) mational freedom;12 (ii) the λmax are intermediate between the longest λabs max values for LPPP (446 nm) and LPPPT (532 nm).12,48 This illustrates that a reduction of the optical excitation energy is achieved by the introduction of thienyl units. In contrast to ladder polymers, the conjugation between neighboring repeat units is somewhat decreased in our step-ladder copolymers due to some rotational freedom of the connecting inter-ring single bonds. However, the corresponding methylene-bridged all-thienylene ladder polymers do not seem to be accessible due to an increased steric hindrance.12 Nevertheless, the synthesis of imine-bridged all-thienylene based ladder polymer have been reported.13 For these, only small redshifts in their absorption maximum (439 nm) were measured when compared to the corresponding ladder poly(phenylenethienylene)s (LPPPT) and the copolymers described here (Table 2).13,28 In that particular case, this was attributed to the arylsubstituents (at the adjacent imine-carbon position) that causes a twisting of the ladder backbone and thus hinders a maximum electron delocalization.13 Conclusions The spectroscopic and photophysical properties of the methylene bridged step-ladder oligothiophene-phenylene and oligothiophene-naphthylene copolymers were evaluated in solution (at 293 and 77 K) and in the solid state (thin films). In general, the excited state decay is dominated by radiationless processes, in particular internal conversion. This was attributed to the “semi-flexible” structure of these copolymers that induces conformational disorder, and consequently enhances the coupling (vibrational and rotational modes) between the excited (S1) and ground state (S0), thus leading to the increase of the internal conversion deactivation channel. Fluorescence decays in the picosecond time domain could only be fitted with a triexponential decay. The rise and decay components seen for lower and higher excitation energies of the emission are associated with the shortest lifetime component and, based on our observations, are attributed to on-chain energy transfer. The distinction between this mode of deactivation (energy transfer) and conformational relaxation was made on the basis of a detailed analysis of the fluorescence decays (covering the entire emission spectra), in different solvents and in the comparison with an oligomeric model compound. It is our belief that this approach could be generalized to differentiate between the contributions of these two modes of deactivation in other conjugated polymers. Acknowledgment. Financial support from FEDER and FCT is acknowledged. J.P. acknowledges FCT for a Ph.D. grant (SFRH/BD/18876/2004). Pulse radiolysis experiments were carried out at the Free Radical Research Facility in the Synchrotron Radiation Department of the CLRC Daresbury Laboratory, Warrington, UK, with the support of the European Commission through the project “Dynamics of Conjugated copolymers and oligomers in solution and solid state” European Commission Transnational Access to Major Research Infrastructures Human Potential Programme. Drs. S. Navaratnam and

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R. Edge (FRRF, Daresbury Laboratory) are acknowledged for their excellent technical support in these. References and Notes (1) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954. (2) Costa, T.; Pina, J.; Seixas de Melo, J. Photophysical processes in polymers and oligomers. In RSC Special Periodic Reports in Photochemistry; Albini, A., Ed.; Vol. 37 (in press). (3) Bunnagel, T. W.; Galbrecht, F.; Scherf, U. Macromolecules 2006, 39, 8870. (4) Goldfinger, M. B.; Crawford, K. B.; Swager, T. M. J. Am. Chem. Soc. 1997, 119, 4578. (5) McCulloch, I.; Heeney, M.; Chabinyc, M. L.; DeLongchamp, D.; Kline, R. J.; Coelle, M.; Duffy, W.; Fischer, D.; Gundlach, D.; Hamadani, B.; Hamilton, R.; Richter, L.; Salleo, A.; Shkunov, M.; Sporrowe, D.; Tierney, S.; Zhong, W. AdV. Mater. 2009, 21, 1091. (6) Scherf, U. J. Mater. Chem. 1999, 9, 1853. (7) Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1998, 120, 5355. (8) Zade, S. S.; Bendikov, M. Chem.sEur. J. 2007, 13, 3688. (9) Setayesh, S.; Marsitzky, D.; Scherf, U.; Mullen, K. C. R. Acad. Sci., Series IV 2000, 1, 471. (10) Jacob, J.; Sax, S.; Piok, T.; List, E. J. W.; Grimsdale, A. C.; Mullen, K. J. Am. Chem. Soc. 2004, 126, 6987. (11) Wong, K. T.; Chi, L. C.; Huang, S. C.; Liao, Y. L.; Liu, Y. H.; Wang, Y. Org. Lett. 2006, 8, 5029. (12) Forster, M.; Annan, K. O.; Scherf, U. Macromolecules 1999, 32, 3159. (13) Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1997, 119, 9624. (14) Bunnagel, T. W.; Nehls, B. S.; Galbrecht, F.; Schottler, K.; Kudla, C. J.; Volk, M.; Pina, J.; Seixas de Melo, J. S.; Burrows, H. D.; Scherf, U. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 7342. (15) Becker, R. S.; Seixas de Melo, J.; Mac¸anita, A. L.; Elisei, F. J. Phys. Chem. 1996, 100, 18683. (16) Pina, J.; Burrows, H. D.; Becker, R. S.; Dias, F. B.; Mac¸anita, A. L.; Seixas de Melo, J. J. Phys. Chem. B 2006, 110, 6499. (17) Pina, J.; Seixas de Melo, J.; Burrows, H. D.; Mac¸anita, A. L.; Galbrecht, F.; Bunnagel, T.; Scherf, U. Macromolecules 2009, 42, 1710. (18) Stricker, G.; Subramaniam, V.; Seidel, C. A. M.; Volkmer, A. J. Phys. Chem. B 1999, 103, 8612. (19) Pina, J.; Seixas de Melo, J.; Burrows, H. D.; Bilge, A.; Farrell, T.; Forster, M.; Scherf, U. J. Phys. Chem. B 2006, 110, 15100. (20) Bensasson, R. V.; Land, E. J.; Truscott, T. G. Excited states and free radicals in biology and medicine; Oxford Science Publications: Oxford, 1993. (21) Murov, S.; Charmichael, I.; Hug, G. L. Handbook of Photochemistry; M. Dekker Inc.: New York, 1993. (22) Burrows, H. D.; Seixas de Melo, J.; Forster, M.; Guntner, R.; Scherf, U.; Monkman, A. P.; Navaratnam, S. Chem. Phys. Lett. 2004, 385, 105. (23) Martinez, C. G.; Neumer, A.; Marti, C.; Nonell, S.; Braun, A. M.; Oliveros, E. HelV. Chim. Acta 2003, 86, 384.

Pina et al. (24) Oliveros, E.; Suardimurasecco, P.; Aminiansaghafi, T.; Braun, A. M.; Hansen, H. J. HelV. Chim. Acta 1991, 74, 79. (25) deMello, J. C.; Wittmann, H. F.; Friend, R. H. AdV. Mater. 1997, 9, 230. (26) Palsson, L. O.; Monkman, A. P. AdV. Mater. 2002, 14, 757. (27) Mahrt, R. F.; Pauck, T.; Lemmer, U.; Siegner, U.; Hopmeier, M.; Hennig, R.; Bassler, H.; Gobel, E. O.; Bolivar, P. H.; Wegmann, G.; Kurz, H.; Scherf, U.; Mullen, K. Phys. ReV. B 1996, 54, 1759. (28) Laquai, F.; Mishra, A. K.; Ribas, M. R.; Petrozza, A.; Jacob, J.; Akcelrud, L.; Mullen, K.; Friend, R. H.; Wegner, G. AdV. Funct. Mater. 2007, 17, 3231. (29) Fonseca, S. M.; Pina, J.; Arnaut, L. G.; Seixas de Melo, J.; Burrows, H. D.; Chattopadhyay, N.; Alcacer, L.; Charas, A.; Morgado, J.; Monkman, A. P.; Asawapirom, U.; Scherf, U.; Edge, R.; Navaratnam, S. J. Phys. Chem. B 2006, 110, 8278. (30) Seixas de Melo, J.; Silva, L. M.; Kuroda, M. J. Chem. Phys. 2001, 115, 5625. (31) Monkman, A. P.; Burrows, H. D.; Hartwell, L. J.; Horsburgh, L. E.; Hamblett, I.; Navaratnam, S. Phys. ReV. Lett. 2001, 86, 1358. (32) Dias, F. B.; Mac¸anita, A. L.; Seixas de Melo, J.; Burrows, H. D.; Guntner, R.; Scherf, U.; Monkman, A. P. J. Chem. Phys. 2003, 118, 7119. (33) Dogariu, A.; Vacar, D.; Heeger, A. J. Phys. ReV. B 1998, 58, 10218. (34) Di Paolo, R. E.; Gigante, B.; Esteves, M. A.; Pires, N.; Santos, C.; Lameiro, M. H.; Seixas de Melo, J.; Burrows, H. D.; Mac¸anita, A. L. ChemPhysChem 2008, 9, 2214. (35) Di Paolo, R. E.; Seixas de Melo, J.; Pina, J.; Burrows, H. D.; Morgado, J.; Mac¸anita, A. L. ChemPhysChem 2007, 8, 2657. (36) De Leener, C.; Hennebicq, E.; Sancho-Garcia, J. C.; Beljonne, D. J. Phys. Chem. B 2009, 113, 1311. (37) Scheblykin, I. G.; Yartsev, A.; Pullerits, T.; Gulbinas, V.; Sundstrom, V. J. Phys. Chem. B 2007, 111, 6303. (38) Di Paolo, R. E.; Burrows, H. D.; Morgado, J.; Mac¸anita, A. L. ChemPhysChem 2009, 10, 448. (39) Forster, M.; Scherf, U. Macromol. Rapid Commun. 2000, 21, 810. (40) Lemmer, U.; Heun, S.; Mahrt, R. F.; Scherf, U.; Hopmeier, M.; Siegner, U.; Gobel, E. O.; Mullen, K.; Bassler, H. Chem. Phys. Lett. 1995, 240, 373. (41) Teetsov, J.; Fox, M. A. J. Mater. Chem. 1999, 9, 2117. (42) Justino, L. L. G.; Ramos, M. L.; Abreu, P. E.; Carvalho, R. A.; Sobral, A.; Scherf, U.; Burrows, H. D. J. Phys. Chem. B 2009, 113, 11808. (43) Seixas de Melo, J.; Pina, J.; Burrows, H. D.; Brocke, S.; Herzog, O.; Thorn-Csanyi, E. Chem. Phys. Lett. 2004, 388, 236. (44) Pina, J.; Seixas de Melo, J.; Burrows, H. D.; Galbrecht, F.; Nehls, B. S.; Farrell, T.; Scherf, U. J. Phys. Chem. C 2007, 111, 7185. (45) Burrows, H. D.; Seixas de Melo, J.; Serpa, C.; Arnaut, L. G.; Monkman, A. P.; Hamblett, I.; Navaratnam, S. J. Chem. Phys. 2001, 115, 9601. (46) Kohler, A.; Beljonne, D. AdV. Funct. Mater. 2004, 14, 11. (47) Monkman, A.; Burrows, D. Synth. Met. 2004, 141, 81. (48) Forster, M.; Pan, J. Y.; Annan, K. O.; Haarer, D.; Scherf, U. Polym. Int. 2000, 49, 913.

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